1. Field of the Invention
The invention pertains to the field of control systems. More particularly, the invention pertains to signal converters for mechatronic control systems.
2. Description of Related Art
In automated mechatronic systems the precise positioning of mechanical loads is predominantly accomplished in small increments of position resolution, i.e., the positioning subsystem has target positions that are quantized such that it can only stop at these quantized positions. These systems are referred to in the art as incremental motor control systems.
The earliest system of this type to gain popularity were systems based upon stepper motors, where the electro-mechanical design of the motor creates natural incremental stopping points that can be transitioned to by cycling or “stepping” current through the motor phase windings, each increment in a phase winding current cycle leading to another incremental step of the output shaft angle, see U.S. Pat. No. 3,506,859. If one desires to move from point A to point B, some incremental number of steps away from point A, the phase winding current cycle simply needs to be repeated the appropriate number of times based upon the distance between A and B. However, the current in the motor phase windings can not be arbitrarily cycled to move from point A to point B without any respect to the timing of the motor phase winding current cycles. These cycles need to be precisely timed in order for the system to operate properly.
An evolution of the stepper motor control system that made it more usable to a wider audience was the introduction of the incremental pulse input stepper motor drive that would advance the motor phase winding current cycle one incremental step for each pulse that was provided to one of its inputs while another input was used to control the direction of motor phase current cycle to cause forward or reverse motion, see prior art FIG. 1 of this application, also U.S. Pat. No. 4,153,866. (Alternate input formats include one input for a forward pulse and another for a reverse pulse, etc.). These incremental pulse input stepper motor drives removed the machine control system of the need to calculate, deliver and control the current in the stepping motor's phase windings to increment the shaft position. The timing of the incremental pulses sent to the incremental pulse input stepper drive's inputs, however, is still important for proper operation of the motor control system. These incremental pulses need to be ramped up and down smoothly in frequency in order to have error free and smooth operation of the motor.
FIG. 1 shows a block diagram of such a prior art system using an incremental pulse input stepper motor drive 1, where a counter or state machine 4 is driven though states by the incremental pulse inputs Step 1 and Direction 2. The output of this counter 4 indexes look-up memory 5 whose output values are used to set the current in the phase windings 8a & 8b via digital-to-analog converters 6a and 6b and transconductance amplifiers 7a and 7b. 
As the state-of-the-art advanced, systems were developed that retained the input interface of the stepper motor drive, i.e., the incremental pulse input described above, but were constructed to drive motors which were not stepper motors (i.e., to drive motors that do not have the natural electromagnetic stopping points). These alternate motors provide increased performance when coupled with a feedback device to form a closed-loop servo motor system. In these closed-loop motor drive systems the incremental pulse input is sent to a counter whose output becomes the reference for the servo control loops.
Prior art FIG. 3 shows an incremental pulse input servo motor drive 30 where a position counter 4 is incremented/decremented by incremental pulse inputs Step 1 and Direction 2. The output of this counter 4 becomes the position reference for a servo control system comprised of a summing node 31, servo compensator 32, an amplifier 33, motor 34 and sensor 35.
Prior art FIG. 2 shows a frequency controlled pulse stream 20 as required for use with a prior art incremental pulse input stepper motor drive (as in FIG. 1) or a prior art incremental pulse input servo motor drive (as in FIG. 3). Resultant velocity vs. time 21 and position vs. time 22 graphs are also shown.
Due to wide adoption and familiarity with the incremental pulse input method, incremental pulse input motor drives are very popular and low cost, both those that drive stepper motors as well as those that drive servo motors. Although servo motor drives with an incremental pulse input offer increased performance over the stepper motor based systems, the physics of these systems still require that the timing of the incremental input pulses be precisely controlled for smooth and error-free motion.
In mechatronic machine systems the timing of the incremental input pulses sent to these motor drives is precisely controlled by a trajectory generator or “indexer” subsystem. Trajectory generators create pulse sequences with precisely controlled frequency ramps during the starting and stopping portions of a move to control the acceleration and deceleration of the motion as shown in prior art FIG. 2.
A trajectory generator produces an incremental pulse command sequence, similar to that shown at 20 in FIG. 2, in response to positioning demands from a control computer that is coordinating the incremental motion of the motors within the mechatronic machine. The structure of a mechatronic axis control system is shown in prior art FIG. 4, which shows a mechatronics axis control system with a incremental pulse input motor drive 42 of the prior art which requires the use of a trajectory generator 41 to control a motor 43.
Many control computers do not have built-in trajectory generators, especially low cost controllers such as micro-PLCs (Programmable Logic Controllers). Some control computers can have trajectory generators added to them at significant cost, or alternately, the trajectory generator function can be constructed with software running within the machine controller, but only with significant effort and often with significant limitations due to the precise timing/frequency ramping requirements.